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### Course: Arithmetic>Unit 13

Lesson 4: Adding and subtracting fractions with unlike denominators

# Subtracting fractions with unlike denominators introduction

Learn how to subtract fractions with unlike denominators. Watch the process of finding a common denominator, converting the fractions, and then performing the subtraction.

## Want to join the conversation?

• why math got to be so hard
• I know it́s super hard but super ez at times
• my question is how would u do it like step by step with divison or multiplication
• You multiply the numerator and denominator across for the fractions you want to multiply
• So you still find a common denominator even if you are subtracting.
• why in some questions i solve by writing out the problem and still get it wrong, but I did it exactly how it shows.
• Check it again
• How do i get the answer wrong?
• Maybe you need help with math? It's ok!
• who is Sal? I like him better than the girl.
• very true
• So isn't Sal just saying that when you subtract fractions that have different denominators ( 1/2 - 1/3) that you just multiply the other denominator to both of the numerators and denominators? Ex. 1/2 = 3/6, 1/3 = 2/6
• Why did you multiply by 3? Was it because it was 1/3 so we have to multiply by 3? For example if we have 1/4, we would have to multiply by 4 right?
• You are partially correct.
It is indeed because of 1 / 3, but not directly related to the denominator.
You might have missed a few lessons if you're raising this question. Check this out:

When we want to find a common denominator between 2 fractions, we find the LCM of the denominators of the 2 fractions.
LCM(2, 3) = 6
Now, we find a number such that when it is multiplied by 2 = 6, which we can solve and find out it is 3.
Hence we multiply by 3.

The same goes to the fraction with 3.

Okay, now let's say we have 1 / 2 and 1 / 4.
LCM(2, 4) = 4
Now, we find a number such that when it is multiplied by 2 = 4, which we can solve and find out it is 2, and not 4!
(1 * 2) / (2 * 2)
= 2 / 4

For 4, we don't have to multiply by anything (Multiply by 1, to be precise).